This invention relates generally to the field of opto-electronic devices. In particular, embodiments of the present invention relate to a transistor header with a printed circuit board inserted perpendicularly through the cylindrical base. For example, embodiments of the present invention may find particular use in conjunction with lasers or optical receivers.
Transistor headers, or transistor outlines, (TOs) are extremely common in the field of opto-electronics. They essentially consist of a cylindrical metallic base with a number of conductive leads extending perpendicularly through both sides of the base. The leads are hermetically sealed to the base provide mechanical and environmental protection for the components contained in the TO package. In addition, the leads are sealed to the base in order to electrically isolate them from the metallic base. The ground lead is often electrically connected directly to the base. Various devices are typically mounted on the device side of the base of the header and connected to the leads. Some form of cap is commonly sealed over the device side of the base to prevent particles from damaging whatever device is mounted on the base. The type of cap and header typically depends on the application and the particular device being mounted on the base of the header. For example, if a semiconductor laser is mounted on the header, a cap would need to be at least partially transparent to allow the optical signal to be transmitted from the TO package. Compared to the shapes of other packages (e.g. the rectangular butterfly package), the cylindrical shape of a transistor header offers great advantages in terms of manufacturing costs and size sensitive adaptability.
Transistor headers are relatively inexpensive and they are manufactured in various configurations. The diameter of the base and the diameter of the leads are both variable. Standard transistor header diameter configurations are given specific numbers such as TO-5 or TO-46. These specific transistor header diameter configurations can still have different numbers of leads and can be equipped with many different caps.
Transistor headers are commonly used to protect and electrically attach sensitive electrical devices to printed circuit boards (PCBs). Specifically sized receptacles can be mounted on PCBs that electrically house a particularly sized header. In order to operate properly, environmentally sensitive devices, such as laser diodes, must be provided mechanical and environmental protection. By sealing the device in a chamber between the base of the header and the cap, the device is protected from mechanical and environmental damage, as well as contamination.
One of the general problems with standard headers is the limited number of potential leads or the small Input/Output (I/O) density. Certain complex devices require many isolated electrical connections to function efficiently. Standard headers are limited to the number of hermetically sealed leads that can be physically positioned on the base. Therefore, to increase the number of leads, the base must be made larger. In most applications, space is limited and it is undesirable to have a large header assembled within an electronic or opto-electronic subassembly.
Semiconductor devices often generate heat during operation and it is important to provide a means for removing thermal energy from the device in order to optimize performance. Standard headers are composed primarily of metallic materials (e.g. kovar) that are not particularly good thermal conductors. The high thermal resistance of most headers requires that special heat dissipation techniques be used to maintain the performance of a device that is mounted on the header. These heat dissipation techniques may require thermoelectric coolers (TECs), specialized thermal films, etc.
Semiconductor lasers operate efficiently if their driving impedance is precisely balanced with the impedance at the terminals. Impedance matching is often accomplished with additional electrical components such as resistors, capacitors and carefully designed transmission lines such as microstrips or striplines. The limited physical space available on standard headers often prevents additional components from being mounted directly on the base of the header and therefore the components must be mounted on the actual printed circuit board. Device performance is compromised due to limitations in the ability to place these components sufficiently close to active devices such as lasers or ICs. The limited number of conductive leads on standard headers further limits the number of additional components that can be used to match the driving impedance with the terminal impedance. Finally, it is relatively difficult to precisely control the electrical impedance of the glass/metal feedthru due to 1, limited choices of the lead diameter; 2) limited choices of the dielectric value of the sealing glass; and 3) difficulty in controlling the position of the lead with respect to the thru hole in the header base.
Therefore, there is a need for a header that increases the I/O density of the standard header without increasing the relative diameter of the header. In addition, the header should provide a larger area to mount additional devices without increasing the overall diameter. The improved header should selectively incorporate higher thermal conductivity paths such that heat generation within the package can be managed more effectively. Such a header would have all of the features of a standard header with the additional advantages mentioned above.
These and other problems in the prior art are addressed by embodiments of the present invention, which relates to an improved header assembly that is equipped with an additional platform extending perpendicularly through the base of the header.
In an exemplary embodiment, the header assembly includes a substantially cylindrical metallic base and a platform extending perpendicularly through and out from either side of the base. The platform is constructed from an insulating material such as a ceramic. The platform is hermetically sealed to the base so as to provide mechanical and environmental protection for the components contained in the TO package. A flat surface formed by the platform on either side of the base is capable of receiving multiple electrical components. The platform includes at least one conductive pathway that extends throughout the platform and consequently through the base. The conductive pathway(s) are capable of conducting isolated electrical signals from one side of the base to the other in substantially the same manner as the leads found on standard headers.
The header assembly includes a ceramic platform that provides several preferred functions. In particular, the ceramic platform can be fabricated with multiple layers and finely defined electrical traces, thus allowing it to house multiple electrical components on either side of the base. In addition, the ceramic platform can be fabricated with materials such as aluminum nitride and beryllia, which have higher thermal conductivity than the base of a standard header. The ceramic platform also increases the electrical input/output density of the header by increasing the number of potential electrical connections between the two sides of the base. For instance, many prior art transistor header assemblies have only four leads, whereas the transistor header assemblies of the invention can have significantly more electrical connections as desired.
A semiconductor laser is one example of the devices that can be mounted on the platform of the header assembly. To operate efficiently, the laser""s driving impedance must be matched with the component impedance. Additional components are mounted on top of the platform and on either side of the base to properly match the impedances. The platform is formed from a ceramic, thereby increasing the thermal conductivity of the header. In addition, a photodiode is mounted directly either in a recessed pocket of the platform or on the side of the base containing the semiconductor laser. The photodiode is used for feedback purposes.
An optical receiver is another example of a device that can be mounted on the platform of the header assembly. An optical receiver can be a photodetector or any other device capable of detecting optical signals. Optical receivers may also require additional electrical components to be mounted on the platform. In addition, optical receivers can be configured to detect optical signals in a vertical orientation or a horizontal orientation. Therefore, an optical receiver can be mounted in either of these orientations on the platform of the header assembly.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.